For the kind of use and exposure to the elements your truck is likely to see, you could leave it bare and it would outlast you.

If I were painting it, I'd use satin black oil based paint over oil based primer. Coat all six sides of the wood prior to installation.

For stain, I'd use oil based, semi-solid Cabot stain. Even with semi-solid, you can still see grain.

You're truck's body is unrestored and beat up. I think original type pine boards, painted black would be more in keeping with the truck's overall original look rather than the incongruous appearance of a shiny oak bed that looks more like a dining room table than a serious work vehicle.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)